In water treatment plants, particularly for large municipalities, alum (aluminum sulphate) is commonly used in the coagulation and clarification process. During the treatment the alum is converted essentially to aluminum hydroxide. The aluminum hydroxide, together with suspended solids including suspended matter in the raw water, precipitates resulting from the treatment process, additives, etc. form a sludge which must be disposed of.
In the past waste products from the water treatment were simply discharged to the nearest body of water. With increasingly stringent anti-pollution standards, however, disposal is troublesome and expensive.
It has previously been proposed to dewater the waste sludge in a filter press, after suitable thickening and treatment with lime, thereby producing cake which can be trucked away and used for land fill, etc. With large amounts of waste, large filter press capacity is required, and new alum supplied as required for the treatment process.
Systems have also been described in which alum is recovered for re-use through acidulation of the aluminum hydroxide in the sludge. This permits recycling the recovered alum so that smaller amounts of new alum are required. Separation of the recovered alum solution from the remaining waste has been effected through the use of settling tanks, and the remaining waste drawn off and thereafter neutralized and dewatered.
During the period of recycling, the acidulation may resolubilize precipitates previously formed during the water treatment process. While there are probably many raw water mineral elements that would react in this manner, iron and manganese are likely to be of principal concern. As recycling proceeds, the iron and manganese accumulates in the recovered alum solution sufficiently to impair the water treatment or at least to become a potential problem. Other undesirable material may also gradually accumulate. It is therefore desirable to eliminate the recovered alum at intervals depending on the composition of the raw water and the treatment conditions, and start over with a new supply of alum.
It is also possible that, from time to time, undesirable wastes will appear in the raw water intake which, if retained in some form in the recycled alum solution, might threaten the quality of the product water. Such wastes might result from spills of industrial waste into the water source, extreme biological activity, flood wash-downs, etc. While likely to be of infrequent occurrence, it is prudent to take such conditions into account in the overall system since continuous operation is important in a municipal supply.
The present invention is directed to the provision of a water treatment system and process which normally utilizes alum recovery and yields a neutral cake for convenient disposal, provides for periodic elimination of contaminated recovered alum, and permits operation without alum recovery when required by abnormal conditions, with due regard to efficiency and cost considerations.